In large bowl, whisk together baking soda, cornstarch, epsom salt, and citric acid until well combined and clump-free.

Place wet ingredients in small mason jar with lid then gently shake until well combined.

Add 1 teaspoon at a time of wet mixture to dry mixture while whisking, if mixture starts to bubble....SLOW DOWN! (Once all liquid is added, mixture will be the consistency of lightly damp sand and will barely clump together.)

Transfer mixture evenly into silicone molds and press down firmly until well packed.

Allow mixture to sit in molds overnight or longer, until completely hardened.